O-320 with a bad cam...rebuild?

tawood

tawood

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Tim
So my plane has been in the shop since April for this problem:

https://www.pilotsofamerica.com/com...al-and-counting-with-not-much-to-show.104012/

The good news / bad news is, my mechanic finally figured out what was wrong: a cam lobe check by measuring the valve lift found my cam is bad (excuse me if I got some terminology wrong from memory).

So, I have an O-320 motor with only 400 hours SMOH that needs a new cam...does this require a total rebuild? BTW, we're not sure why the cam went bad, as the engine has been used regularly since rebuild, I add cam guard, etc.
 
Define "total rebuild." The case will need to be split. What needs to be done to put it back together correctly depends on what was done to it 400 hours ago. I demand new (not reground) cams. I'd prefer the case to be freshly milled and align bored before installing the new cam. At that point i'd rather use new bearings so the crank may as well get ground and replated. And the dominoes fall. Me? I'd do the rebuild to major overhaul requirements using factory new clearance tolerances. Others will surely disagree. Do your own research and make your own decision.
 
Stewartb said:
Define "total rebuild." The case will need to be split. What needs to be done to put it back together correctly depends on what was done to it 400 hours ago. I demand new (not reground) cams. I'd prefer the case to be freshly milled and align bored before installing the new cam. At that point i'd rather use new bearings so the crank may as well get ground and replated. And the dominoes fall. Me? I'd do the rebuild to major overhaul requirements using factory new clearance tolerances. Others will surely disagree. Do your own research and make your own decision.
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Thanks...I've been trying to do research, but I'm not finding much out there, other than the fact you mentioned about splitting the case...oh, and that a new cam isn't much more than reground. Seems opinions vary greatly on the new vs reground cam debate.
 
You need to research your engine history. How many runs and what was done during previous overhauls? It may be 400 hours since machined or it may be 4000 hours. Time in service and time since repaired/recertified should play into your decision.
 
Its on its first overhaul, and 2500 TT. I'd have to check the logs for the rest.
 
tawood said:
Its on its first overhaul, and 2500 TT. I'd have to check the logs for the rest.
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Replacing the cam requires nearly complete disassembly. I would make darn certain that is actually the problem by pulling the cylinder with the affected valve lift issue then inspecting the cam.

I'm a bit skeptical of the diagnosis especially if the screen and oil filters are clean. How would reduced lift increase EGT by 250 degrees?
 
bnt83 said:
Replacing the cam requires nearly complete disassembly. I would make darn certain that is actually the problem by pulling the cylinder with the affected valve lift issue then inspecting the cam.

I'm a bit skeptical of the diagnosis especially if the screen and oil filters are clean. How would reduced lift increase EGT by 250 degrees?
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Being done today...my mechanic is skeptical as well.
 
tawood said:
Its on its first overhaul, and 2500 TT. I'd have to check the logs for the rest.
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It doesn't "need" an OH....just a repair to fix the cam issues.

If it were me I'd get the estimates and make this decision purely on economics....it isn't a safety issue. IMHO.
 
bnt83 said:
Replacing the cam requires nearly complete disassembly. I would make darn certain that is actually the problem by pulling the cylinder with the affected valve lift issue then inspecting the cam.

I'm a bit skeptical of the diagnosis especially if the screen and oil filters are clean. How would reduced lift increase EGT by 250 degrees?
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I'm just spitballing for the sake of spitballing, but perhaps a reduced exhaust valve lift would increase compression on the exhaust stroke sufficiently to increase the temperature that far? Or restriction in the intake valve enough to lean it to show that kind of increase?

I dunno. :dunno:
 
Grum.Man said:
could be a flat lifter instead of a worn cam lobe. I wouldn't overhaul anything as long as it all looked good when you split the case. Just put new bearings and a gasket kit in and put it back together.
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and....if so, a tap changing with RPMs will be present.
 
Doesn't bad lobes usually indicate the hardened surface is shot due to corrosion or excessive wear? Wouldn't that put metal in the oil to circulate through the bearings and damage other components?
Too bad the OH shop 400 hours ago doesn't stand behind the cam.
 
I would check the logs for what was done at the rebuild. If the crank was ground and the case was surfaced, I would be inclined to disassemble, replace the cam and followers, and reassemble assuming the crank and main bearings show minimal wear.
 
kyleb said:
I would check the logs for what was done at the rebuild. If the crank was ground and the case was surfaced, I would be inclined to disassemble, replace the cam and followers, and reassemble assuming the crank and main bearings show minimal wear.
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And see if there's evidence of the wrong parts being put in. 400 hours seems way too low for this. Depending on today's findings it's almost a guarantee I'd want that case split to see why and whether other damage was done.
 
my money is on a MIF....maintenance induced failure....with that few hours.
 
Just for the sake of conversation, let's suppose that the cam has 2500 hours on it. It was removed, and inspected, but the folks that did the OH,(400 hours ago) and was found to be in good shape, "no need to send it off, let's put it back in". Point is that there are certian parts that are required to be replaced at OH, and I don't think the cam is one of them.
Another scenerio, could be the cam was replaced with a used servicable one with who knows how many hours on it.
Or it could well have been replaced with brand spankin' new one from Lycoming, and it just wasn't up to snuff.

I would hazard to guess that the case was the first that I mentioned. Who did the OH?
 
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